1. Field of the Invention
The present invention relates to color image processing methods and apparatus for eliminating errors associated with multigeneration color image reproductions. More particularly, the present invention relates to image processing methods and apparatus for eliminating multigeneration color image reproduction errors in color reproduction devices such as color facsimile machines, color photocopiers, and the like.
2. Discussion of the Related Art
U.S. Pat. No. 4,974,071 teaches an apparatus for encoding color image data. Further, U.S. Pat. No. 4,941,038 discloses a method for color image processing.
FIG. 1 shows a conventional image reproduction apparatus. The apparatus includes an image input device 1, for inputting an image to be reproduced, and an image output device 7, for outputting an image reproduction corresponding to the input image. The function of the apparatus is to output an image reproduction which corresponds as closely as possible to the image input by image input device 1.
When reproducing an image (e.g., when making a photocopy of an image displayed on an original document), the image must first be input into the reproduction apparatus. This input function is typically carried out using a standard image input device 1. Various types of image input devices are known. For example, the image can be input using a scanner (as in a photocopier device), or it can be built up by an operator on a CRT and then input from the CRT according to known principles.
After being input, the image (i.e., data representing the image) is stored in an image buffer 3. The image buffer is generally a rewritable memory device such as a RAM (random access memory). If the image to be stored is a color image, then RGB (red, green, blue) values are produced by the image input device 1 and stored in image buffer 3 as the image representation. Alternatively, the image input device 1 could produce L*a*b* (lightness, red-green axis, yellow-blue axis) color space coordinates representing the image to be stored in the image buffer 3. A further alternative would be to convert RGB values produced by the image input device 1 into L*a*b* color space coordinates, and then store the resulting L*a*b* color space coordinates in the image buffer 3 as the image representation.
The representation of a color image using RGB values and L*a*b* color space coordinates is well known, as evidenced by the following articles written by P. Laihanen, incorporated herein by reference: "Optimization of Digital Color Reproduction on the Basis of Visual Assessment of Reproduced Images," Proceedings of the SID, Volume 30, No. 3, pp. 183-190 (1989), and "Color Reproduction Theory Based on the Principles of Colour Science," Advances in Printing Science and Technology, June 1987 Conference, Pantech Press, London (1988). Conversion between RGB values and L*a*b* color space coordinates is also well known.
After storing the image in the image buffer 3, the image is subjected to color correction processing by color correction circuitry 5. The color correction circuitry 5 commonly takes the form of a calibrated device for converting the image data stored in the image buffer 3 to a form that will allow the image output device 7 to output a faithful reproduction of the image input to the image buffer 3 from image input device 1. For example, the color correction circuitry could be a device for converting the image data output from image buffer 3, which is in the form of RGB values or L*a*b* color space coordinate values, into CYM (cyan, yellow, magenta) values, which are applicable for use in the image output device 7.
In apparatus such as the image reproduction apparatus shown in FIG. 1, it is often the case that differences exist between the color characteristics of the image input by the image input device 1 and the color characteristics of the reproduced image output by the image output device 7. Such color errors are usually measured in L*a*b* color space because it is perceptually uniform (e.g., two greens separated by a difference of one unit in L*a*b* color space look as different as two reds separated by the same distance in L*a*b* color space). In L*a*b* color space, the L* axis represents lightness, the a* axis is the red-green axis, and the b* axis is the yellow-blue axis. An example of the representation of a color image in L*a*b* color space is shown in FIG. 2. As can be seen from FIG. 2, the colors contained within the image lie within an identified volume 100, which is called the color gamut of the image.
With reference to FIG. 2, it is known that a color difference as small as one or two L*a*b* units is just noticeable under ideal conditions (e.g., where low noise colors are positioned adjacent to each other). However, much larger errors are acceptable between an original image to be reproduced and the reproduction of the image (e.g., a photocopy reproduction of the image). For example, opaque copies made of opaque originals using photographic techniques generally have RMS (root mean square) color errors of about 15 L*a*b* units, where the average is taken over the set of colors found in standard color checker apparatus. Zerographic digital copiers are known to perform much better. For example, various models of zerographic digital copiers have been known to have color errors in the range 7 to 12 L*a*b* units.
In multigeneration copying (i.e., the making of copies using copies as originals), however, the color errors generally build up with each generation of copies. For example, in xerographic digital copiers, the RMS error between the original and a third generation copy (i.e., a copy of a copy of a copy of the original) is 25 to 30 L*a*b* units, which is a very large fraction of the distance from one side of the useful region of L*a*b* color space (about 100 L*a*b* units) to the other. As a result, a third generation copy is usually an unacceptable reproduction.
A major advantage that black and white image reproduction apparatus have over color image reproduction apparatus is that black and white image reproduction apparatus are capable of making high quality reproductions of reproductions (i.e., high quality copies of copies). High quality multiple generation copies can be made of high contrast black and white originals (as opposed to gray originals) in black and white image reproduction apparatus because the images of such originals can be easily quantized. Nevertheless, because it is difficult to quantize color originals, multigeneration reproductions or copies produced by color image reproduction apparatus are low in quality (i.e., high in errors) as compared to miltigeneration reproductions produced by black and white color image reproduction apparatus.